1. Field of the Invention
The invention relates to a memory element and method for fabricating the same, and more particularly to a phase change memory element and method for fabricating the same.
2. Description of the Related Art
A phase change memory (PCM) device may potentially serve as a 64-megabyte (MB) or greater stand-alone non-volatile memory. Before PCM devices become a mainstream replacement for flash memory, however, they must first achieve excellent electrical and thermal performance. Fabrication of non-volatile memory with relatively higher device density using the conventional fabrication process is, thus, a major aim of researchers.
In general, two methods have been employed to increase the storage capability of a phase change memory. First, the amount of memory elements for a single phase change device is increased (i.e. integration increase). Second, multi-bit value in a single memory element is stored.
Since integration of a phase change element has been limited due to the resolution limit of the photolithography process, it is difficult to increase the integration of the phase change element without additional process complexity.
U.S. Pat. No. 6,927,410 B2 discloses a memory device 18 with discrete layers of phase change memory material to introduce multi-bit value into a single memory element. Referring to FIG. 1, the phase change element includes a bottom electrode 28, a top electrode 26, a plurality of phase change material layers 22 formed on the bottom electrode 28, and a plurality of dielectric layers 24, wherein the dielectric layer 24 is formed between each two adjacent phase change material layers 22 and is separated therefrom. Particularly, the plurality of phase change material layers 22 and the plurality of dielectric layers 24 include a multi-bit programmable structure 20. Referring to FIG. 2, the phase change material layers 22 have been crystallized to form different degrees of crystallization areas 30 by applying various electrical current paths (different pulse amounts or different maintaining time) via the bottom electrode 26 and 28, resulting in exhibition of different resistivities (the crystallization area has less resistivity than that of the amorphous area of the phase change material). Therefore, it is possible to program the phase change memory material with varying degrees of amorphousization/crystallization to produce varying degrees of resistivities, so that more than two possible bit values can be stored in a single memory cell (multi-bit storage).
Programming via crystallization can be performed using a plurality of discrete, shorter crystallizing thermal pulses (with the same or different energy) where the number of such pulses dictates the amount of memory material, and thus the number of phase change material layers, that are crystallized. The phase change material is crystallized in a piece-wise manner, layer by layer, until the desired number of layers is crystallized.
However, in order to fabricate the memory device 18 with multi-bit storage capacity, different strengths or different numbers of crystallizing thermal pulses should be provided, resulting in a necessity to form an additional controller to couple each memory element. Further, due to resistivity drift of the crystallized phase change material, faults may occur in the memory element during operation, thus resulting in lost stored data.
Moreover, there are many layers (such as a plurality of phase change material layers and a plurality of dielectric layers) alternately formed for fabricating the multi-bit memory element, thereby increasing the cost and process complexity and reducing production yield.
Therefore, it is necessary to develop a multi-bit phase change memory to solve the previously described problems.